Suntao Wang

Systematic approach to electrostatically induced 2D crystallization of nanoparticles at liquid interfaces

We report an experimental demonstration of a strategy for inducing two-dimensional (2D) crystallization of charged nanoparticles on oppositely charged fluid interfaces. This strategy aims to maximize the interfacial adsorption of nanoparticles, and hence their lateral packing density, by utilizing a combination of weakly charged particles and a high surface charge density on the planar interface. In order to test this approach, we investigated the assembly of cowpea mosaic virus (CPMV) on positively charged lipid monolayers at the aqueous solution surface, by means of in situX-ray scattering measurements at the liquid–vapor interface. The assembly was studied as a function of the solution pH, which was used to vary the charge on CPMV, and of the mole fraction of the cationic lipid in the binary lipid monolayer, which set the interface charge density. The 2D crystallization of CPMV occurred in a narrow pH range just above the particles isoelectric point, where the particle charge was weakly negative, and only when the cationic-lipid fraction in the monolayer exceeded a threshold. The observed 2D crystals exhibited nearly the same packing density as the densest lattice plane within the known 3D crystals of CPMV. The above electrostatic approach of maximizing interfacial adsorption may provide an efficient route to the crystallization of nanoparticles at aqueous interfaces.

Structure and interaction in 2D assemblies of tobacco mosaic viruses

We created two-dimensional (2D) assemblies of tobacco mosaic viruses (TMVs) and characterized their structures using Atomic Force Microscopy (AFM) and X-ray scattering. The TMVs were adsorbed on an oppositely charged, fluid lipid monolayer supported by a solid substrate and submerged in a buffer solution. The lipid monolayer confined the viral particles within a plane, while providing them with lateral mobility so that overall the TMV assembly behaved like a 2D liquid. We controlled the inter-particle interaction by adjusting the chemical condition in the buffer to induce ordered TMV assemblies. We found that the presence of the lipid layer was essential for forming ordered TMV assemblies. Packed TMV assemblies formed on the lipid layer, with an average inter-particle spacing of 42 nm. By introducing Ca2+ ions into the buffer solution, we were able to improve the in-plane order within the TMV assemblies and reduce the average inter-particle spacing to 20 nm, compared to the TMV diameter of 18 nm. Quantitative analysis of the X-ray scattering data shows that the structural order within the TMV assemblies prepared under a Ca2+-free buffer solution is consistent with purely repulsive, electrostatic inter-particle interaction. In contrast, the structural order within Ca2+-induced TMV assemblies is consistent with the behavior of a fluid of sticky rods, implying the presence of a strong attraction between TMVs. In addition to the screening of Coulomb repulsion, this behavior is likely the result of counterion-induced as well as membrane-mediated attractions.

Role of Electrostatic Interactions in Two-Dimensional Self-Assembly of Tobacco Mosaic Viruses on Cationic Lipid Monolayers

We explore two-dimensional self-assembly of tobacco mosaic viruses (TMVs) on a substrate-supported, fluid lipid monolayer by manipulating the electrostatic interactions, with specific focus on the effects of the cationic lipid concentration in the monolayer and the presence of Ca(2+) ions in the surrounding bulk solution. The TMV assemblies were characterized by grazing-incidence X-ray scattering and atomic force microscopy, and the inter-particle interaction quantified through X-ray scattering data analysis. In the absence of Ca(2+) ions, we found that higher charge densities on the lipid monolayer led to poorer in-plane order, which may be attributed to faster adsorption kinetics, due to the surface potential that increases with charge density. At the same time, higher lipid-charge densities also resulted in weaker repulsion between TMVs, due to partial screening of Coulomb repulsion by mobile cationic lipids in the monolayer. The lipid-charge dependence was diminished with increasing concentration of Ca(2+) ions, which also led to tighter packing of TMVs. The results indicate that Ca(2+) ions strengthen the screening of Coulomb repulsion between TMVs and consequently enhance the role of attractive forces. Control experiments involving Na(+) ions suggest that the attractive inter-TMV interaction has contributions from both the van der Waals force and the counter-ion-induced attraction that depends on ion valence.